termio: fix "forced" shell integration

When a shell is forced, we would supply its /-prefixed executable name
to mimic a path location. The rest of the integration detection logic
assumes just a base executable name. Fix the forced names accordingly.

Also add a unit test for this "force shell" behavior.

src/termio.zig

@@ -16,4 +16,6 @@ pub const Impl = Exec;
 
 test {
     @import("std").testing.refAllDecls(@This());
+
+    _ = @import("termio/shell_integration.zig");
 }